Tin-coated chromium iron alloy and method of making the same



Patented Sept. 23, 1930 UNITED STATES PATENT OFFICE LOUIS SGH'ULTE, 0F PITTSBURGH, PENNSYLVANIA, ASSIGNOR TCl ALLEGHENY STEEL COMPANY, OF BBACKENRIDGE, PENNSYLVANIA, A. CORPORATION OF PENNSYL- VANIA TIN-COATED CHBOMIUH IRON ALLOY AND METHOD OF MAKING THE SAME Ho Drawing. Application filed Kay 25,

This invention relates to the electrodeposition of a sanitary metal, such as tin, on articles made from ferrous iron chromium alloys of the type of rustless iron, stainless steel and the like which contain chromium in the proportions of about 10% or 12%.

While such alloys are to a large extent stainless and rustless when hardened, tempered and polished, they have a less degree of stainlessness or rustlessness when not hardened, tempered and polished.

There is a large and growing demand for stainless and rustless polished and mat surfaced sheets made from alloys of this type for use in the manufacture of articles requiring deep stamping operations, but for such operations, the Brinell hardness should be under 350 and alloys of this type having a Brinell hardness under 350 do not have the requisite degree of stainlessness and rustlessness.

An object of this invention is to produce articles such as sheets which are especially suitable for deep stamping operations, for drawing and other workin and which are stainless and rustless to a high degree.

A further object of this invention is to provide a method of surfacing alloys of this type in a way such as to materially increase the stainlessness and rustlessness of articles made therefrom to an even higher degree than possessed by this type of alloy when hardened, tempered and polished and as heretofore urnished.

These, as well as other objects, which will be apparent to those skilled in the art to whic this invention is directed, I attain by means of the product and the manner of ma king the same described in the specification and particularly pointed out in the appended claims.

Broadly, my. new roduct consists of a chromium iron alloy use to which a dense, hard coating of tin 18 applied b electrodeposition. The alloys to which t 's'invention broadly relates are those in which the chromium content is around 12% and the carbon content is not over 14%, but a chromium content of between 9% and 16% is considered around 12%. The carbon content can be any- 1926. Serial No. 111,626.

thing below 1%. A low carbon content is desirable and necessary when the product is to be subjected to deep stamping operations. 12% carbon has been found satisfactory in many cases, but an even lower carbon content is often desirable.

The chromium iron alloy base may be pro duced by any desired method, but I prefer to produce the same in an electric furnace of the arc type and to have the alloy as free as possible from oxides and occluded ases.

After the alloy is worked up into sheets, plates or other forms and after it has been treated to remove the scale, and is ground or sand blasted and in some cases polished, it is electroplated with tin.

Before plating, however, it is imperative that the surface to be plated be rendered passive as no satisfactory deposit of tin or other sanitary metal can be obtained unless this is done. In order to render these surfaces passive, I immerse the article to be plated in a solution (preferably warm) of nitric acid which I find can range from a 2% solution to a concentrated solution. I find that a concentrated solution is preferable and that it is only necessary to immerse the articles for a few minutes.

In some way, not known to me, the articles become what I term passive and I find that while. the surfaces of the articles remain smooth and polished, if polished before im- Inersion the plating metal will adhere tenaciously which is not the case unless the sur face is rendered passive.

I find that it is necessary to use an electroplating process which will not produce a rough, spongy or tree forming dc osit and I prefer. therefore, to use a so ution of potassium fluoride and tin, as. set forth in United States Letters Patent No. 1,423,686, issued to me on July 25, 1922, and as set forth in said atent, I refer to use from 1 to 2 volts an to have rom 10% to 20% of anode surface in proportion to cathode surface since I have found that with these proportions, the solution with a slight acid reaction, is practically self sustaining. In preparin the plating solution, I dissolve, as set fort in said Patent No. 1,423,686 one pound of canstic potash (KOH) in about two quarts of water. To this, I add partly diluted hydrofluoric acid (HF) until acid reaction is obtained, then add sufiicient water to make from one to ten allons in all. In this solution of potassium uoride having a slight excess of hydrofluoric acid, I sus nd metallic tin as an anode or positive bus ar and on the negative'bus bar I connect metal strips surrounded by porous cups and with the aid of the electric current I dissolve the metallic tin until the solution has absorbed a proximate- 1y one ounce of tin per gallon of aid. I then remove the porous cups and metal strips and suspend the chromium iron alloy articles to be plated. Instead of using the electric current in connection with porous cups for dissolving and saturating a new solution, I may add any soluble tin salt for instance, chloride of tin to the solution.

If it is necessary to have a high lustre or polish on the finished articles, the articles,

before being surfaced with tin must be ground and polished in the same manner that articles are prepared today for nickel plat ing. If only a semi-white lustre or mat finish is required, the articles before being surfaced with tin should be ground or sand blasted and it is advisable after the articles have been surfaced, to scratch brush the same. This treatment of scratch brushing produces an even metallic lustre and if a higher lustre is desired, this can be produced 1 by tumbling the articles in what is commonly known as a ball burnisher.

In order to make the tin coat or surface dense and non-porous and in order tocause the tlfltd alloywith the base metal or alloy, it is necessary to heat treat the articles after the tin has been deposited and before the articles are finally finished. This can be done in various ways. One way is to place the articles in a furnace and raise the same to a tempera ture above the melting point of tin for a short time. I hev'efound t at by raising the coated articles for a short time to a temperature above the melting point of tin, the tin shrinks and changes its structure and I believe alloys with thebase metal or alloy therebeneath. The shrinka of the tin under this heat treatment is visib e to the naked eye.

The timerequired for hi heat treatment depends upon the degree of heat and the weight of the article to be treated. 'A light uge sheet whichhas beentinned, can be eat treated by aub'ectin it to' a temperature of from 1000f for fromone to yalsoteheattrated The tinned articles by lmmersmg them in a neutral chemical compound having a melting point higher than that of tin, as disclosed in United States Patent No. 1,415,198 issued tome on May 9, 1922. TlllS chemical compound may ordinary be manufactured in large quantities they may be satisfactorily heat treated by passingthem between smooth metal rollers raised to a temperature above the melting point of tin.

Having thus described my invention, what I claim is 1. The method which consists in subjecting the surfaces of an article made from an iron chromium alloy to the action of a solution of nitric acid in electrodepositing tin on the surfaces so treated and then in hardening the deposited tin.

2. The method which consists in rendering the surfaces of an iron chromium alloy article passive, in plating the surface with tin and then in hardening the plating metal.

3. The method which consists in passivifying the surface of an iron chromium alloy article, in electrodepositing a sanitary metal on the passivified surface and then in hardening the deposited metal.

4. The method which consists in passivifying the surface of an iron chromium alloy article, in depositing a sanitary metal on such passivified surface and then in raising the temperature of the article above the melting point of the sanitary metal.

5. The method for producing a non-porous adherent deposit of tin on the surface of an iron chromium alloy, which consists in passivif 'ing the surface of such an alloy, in electroepositing tin on the passivified surface and then in heating said plated alloy in a furnace having a temperature higher than the melting point of tin. I

6. The method of producing anon-porous adherent deposit of tin on chromilim iron alloys'which consists in passivifying said alloys in a 2% solution of nitric acid, in electrodepositing a thin layer of tin on said alloy and in hardening said layer of tin by heating to a temperature above the melting point of adherent deposit of tin on chromium iron alloys which consists in passivifying said alloy in a solution rangin from 2% to concentrated nitric acid, m e ectro-depositing a'thin layer of tip on said passivified alloy and in heat treating said layer of tin- '8. The method of producing a non-porous adherent deposit of tin on chromium alloys which consists in passiviflying said alloys, 111 electro-depositing a thin ayer of tin on said alloy and then hardening said plated articles by subjecting to a temperature of from 1,000to 1,500 F. for at least one minute.

9. The method of producing a non-porous adherent deposit of tin on chromium allo s WhlCh consists in passivifying said alloys, n

electro-depositing a thin layer of tin on said alloy and then in hardening said plated articles by subjecting to a temperature of from 1,000 to 1,500" F. for about three minutes.

-10. The method of shrinking and hardening electrolytic tin without the aid of fluxes which conslsts in depositing a thin layer of tin on a passivified base metal and in heating the plated metal in a furnace having a temperature higher than the melting point of tin; said layer of tin being of such thinness that on melting the tin will remain in suspension on the base metal. 11. The method of hardening and shrinking electrolytic tin without the aid of fluxes whi consists in heat treating the plated metal in a furnace havin a temperature above the melting point of tin and in holding the plated metal at that temperature for about three minutes; said electrol ic tin being of such thinness that the tin w 1 not flow on bein melted.

n testimony whereof, I have hereunto subscribed my name this 13th day of Ma 1926.

LOUIS SCHUlZTE. 

